1. Field of the Invention
The present invention relates to an electric injection molding machine, and more particularly to an electric injection molding machine which controls pressures, such as injection pressure, molding clamping pressure of a mold clamping mechanism, product knocking-out pressure in an ejector mechanism, by controlling speed of corresponding servo motors.
2. Description of the Related Art
In an injection molding machine, pressure such as injection pressure and back pressure has been generally feedback-controlled. In such pressure control of the injection molding machine, there may be cases where external noises are superimposed on a detection pressure signal to cause instability to the pressure control.
A control method of performing stable control with reduced noise by means of a filter is disclosed in Japanese Patent Application Laid-Open No. 2001-47480 or No. 2001-239564.
On the other hand, in an electric injection molding machine, the driving source of a movable member is an electric motor such as a servo motor. Pressure control of an object that is driven by a servo motor is also performed usually by controlling the speed of the servo motor or a movable member driven by the servo motor. For example, an injection process is controlled using a pressure loop control. The pressure loop control is intended to control the injection pressure, in which a pressure sensor detects resin pressure within a heating cylinder, pressure within a mold, or pressure to be applied to an injection screw by the resin within the heating cylinder; deviation between the actual detection pressure and the set pressure is determined; a speed command is determined based on the deviation; speed loop control is performed such that speed (speed of servo motor or injection screw) detected by a speed detector or the like coincides with the speed command; and finally the pressure loop control is performed such that the detection pressure coincides with the set pressure.
In such a pressure control system having a speed control loop in the pressure control loop in an electric injection molding machine as described above, it has been necessary to make the speed loop gain substantially low such that no oscillation occurs because of a resonance frequency of a control object. As a result, the speed stability during pressure control becomes worse, resulting in such a problem that stably controlled pressure cannot be obtained with an intended manner.
FIGS. 9 and 10 show detection injection pressure waveform and actual speed waveform of a servo motor (screw) which have been measured when injection pressure control is applied to an electric injection molding machine in which the servo motor drives the injection screw in the axial direction to carry out injection. Reference character a denotes set injection pressure; b, detection injection pressure; and c, actual speed of the servo motor. In this respect, this injection pressure control is to perform pressure loop control based on deviation between the set pressure and pressure detected by a pressure sensor for determining a speed command, to perform speed loop control based on speed deviation between the speed command thus obtained and actual speed of the servo motor detected by a speed detector for determining a current command (torque command) to the servo motor, and to drivingly control the servo motor in accordance with the current command obtained.
FIG. 9 shows detection injection pressure waveform b and actual speed waveform c of the servo motor (screw) when the speed loop gain is decreased in order to suppress resonance of the control object. On the other hand, FIG. 10 shows detection injection pressure waveform b and actual speed waveform c of the servo motor (screw) when the speed loop gain is increased.
In case where the speed loop gain is low as shown in FIG. 9, the response is slow at the time when the set pressure a changes, and as regards both the detection injection pressure b and the actual detection speed c, continuously unstable vibration with overshoot and undershoot with a long period occurs.
On the other hand, in case where the speed loop gain is increased as shown in FIG. 10, the response is good, but as regards both the detection injection pressure b and the actual detection speed c, unstable vibrations with a short period occurs.